High-efficiency battery equalization for charging and discharging

ABSTRACT

A non-contiguous group of cells in a battery of cells is selected for charging or discharging the battery.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of, and incorporatesherein by reference in its entirety, U.S. Provisional Patent ApplicationNo. 61/237,993, which was filed on Aug. 28, 2009.

TECHNICAL FIELD

Embodiments of the invention generally relate to rechargeable batteriesand, in particular, to efficiently charging and discharging cells in abattery.

BACKGROUND

Many different applications, from consumer electronic devices tovehicles and power grids, use rechargeable batteries as energy sources.Typically, these applications require their batteries to be as safe andas efficient as possible. Overcharging a battery, for example, maycompromise its reliability and reduce its useful lifetime. Morealarmingly, over-discharging a battery can cause it to catch fire oreven explode. Thus, any application that charges and/or discharges abattery typically includes circuitry to prevent overcharging and/orover-discharging.

Protecting a battery against these undesirable conditions, however, maybe a complicated task. Many batteries, especially high-voltage orhigh-power batteries, are composed of a plurality of discrete cells,each of which is separately monitored. As more cells are included in thebattery (e.g., to increase its voltage or power output), the chance ofone cell performing at a level different from the other cells in thebattery increases. For example, manufacturing defects or variations maycause each cell in a battery to have a different voltage output, chargecapacity, maximum charging voltage, and/or minimum discharging voltage.Even perfectly matched cells may, over time, become mismatched withusage and age. The worst-performing cell may act as a “weak link” in thebattery; an overcharge-protection circuit, for example, may haltcharging of the battery when the worst-performing cell reaches itslimit, despite the additional capacities of other cells in the battery.

Circuitry may be added to the battery to charge and/or discharge thecells to their individual capacities. This circuitry, however, mayreduce a battery's efficiency and/or significantly increase its cost orcomplexity. A charge-shunting circuit, for example, dumps excess chargeinto resistors to protect battery cells. This charge dumping, however,generally wastes energy and produces heat. A switched-capacitor orflying-capacitor circuit is typically more efficient, but requires theuse of a complicated, costly, and large capacitor and transformernetwork. Still other circuits allow a subset of electrically adjacentcells in a battery to be charged and/or discharged, but require acomplicated, expensive switching network and, because only adjacentcells may be chosen, perform sub-optimally when, for example, cells inthe middle of the battery fail or degrade.

A need therefore exists for a robust, efficient, and flexiblebattery-charging and -discharging system to optimize the charging anddischarging of individual cells in a battery—for example, capable offully charging and discharging any cell in a battery, regardless ofvariations therein, while protecting the cells from overcharging andover-discharging.

SUMMARY

Embodiments of the present invention feature a system that selects asubset of cells in a battery for charging and/or discharging in a mannerthat ensures optimal utilization of all cells, while accounting forcell-by-cell variation. The selected cells may be contiguous (i.e.,electrically adjacent to each other) or non-contiguous (i.e., separatedelectrically by at least one other cell). By selecting the least-chargedcells (when charging the battery) or the most-charged cells (whendischarging the battery), each cell is charged or discharged accordingto its individual capacity, and over time, the cells in the battery areoptimally utilized and not overcharged or over-discharged. The cells maybe monitored during use (e.g., during charging or discharging) and thesubset of cells may be changed to reflect the changing state of thecells. By such time-cycling of the charge and discharge loads across thecells, no lossy and/or expensive charge-distribution circuitry isrequired. Under-performing or failed cells may be bypassed, preservingthe usefulness of the battery while allowing the battery capacity todegrade gracefully.

In general, in some aspects, embodiments of the invention featuremethods for charging (or discharging) a battery of cells. The methodsinclude determining a voltage of each of a plurality of cells in thebattery and selecting a subset of the plurality of cells based at leastin part on the determined voltages. A series connection ofnon-contiguous cells may then be created. The series connection of cellsincludes the subset of cells. The series connection of cells may then becharged with a charging voltage (or discharged into a load).

In various embodiments, selecting the subset of cells includesdetermining at least one of a resting voltage, impedance, or state ofcharge of the cells. The series connection of cells may include at leasttwo adjacent cells. An operating voltage of the series connection ofcells may be approximately equal to the charging voltage (or to adesired voltage across the load), and the charging voltage may bemodulated with a switching circuit. The voltage of each of the pluralityof cells may be determined again after a delay and, based on the newvoltage determinations, a new subset of the plurality of cells may beselected. The voltage of each cell in the series connection of cells maybe monitored during charging (or during discharging). Cells in theseries connection of cells may be mismatched and a parameter (e.g.,resting cell voltage, cell impedance, and percentage of charge) of eachcell may deviate, during charging (or discharging), by less than 1%across the cells. In addition, a faulty cell in the battery may beidentified and bypassed.

In general, in another aspect, a system for managing charge on cells ina battery includes a voltage-sensing circuit and a processor. Thevoltage-sensing circuit senses a voltage across each of a plurality ofcells. The processor selects a subset of the plurality of cells based atleast in part on the sensed voltage. The processor further causes asubset of the plurality of cells to be electrically coupled in anon-contiguous series connection. The system also includes an outputport across the series connection of cells.

In various embodiments, selecting the subset of the plurality of cellsis further based on a resting voltage, impedance, or state of charge ofeach of the plurality of cells. The voltage-sensing circuit may includea voltage sensor local to each cell. Each of the plurality of cells mayfurther include circuitry for disabling the cell if the sensed voltagedeviates from a threshold, and/or a bypass switch. The processor mayfurther include circuitry for selecting, after a delay, a new subset ofthe plurality of cells. A pulse-width modulator may adjust a voltageacross the output port, which may be applied to a voltage source forcharging the series connection of cells or to a load for discharging theseries connection of cells.

In general, in yet another aspect, a system for managing charge on cellsin a battery includes a plurality of cells, each in parallel with abypass switch and in series with a cutoff switch. A voltage-sensingcircuit senses a voltage across each of the plurality of cells. Aprocessor selects a subset of the plurality of cells based at least inpart on the sensed voltages, and configures, using the bypass and cutoffswitches, a series connection of cells comprising the subset of theplurality of cells.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent and may be better understood byreferring to the following description, taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a schematic illustration of a safe cell in accordance with oneembodiment of the invention;

FIG. 2 depicts an exemplary implementation of the safe cell illustratedin FIG. 1 in accordance with one embodiment of the invention;

FIG. 3 depicts a string of safe cells in accordance with an embodimentof the invention;

FIG. 4 depicts a flowchart for charging a string of safe cells inaccordance with an embodiment of the invention:

FIG. 5 depicts a flowchart for discharging a string of safe cells inaccordance with an embodiment of the invention;

FIG. 6 depicts a string of safe cells having voltage regulation inaccordance with an embodiment of the invention;

FIG. 7 depicts an exemplary charging sequence for a string of safe cellsin accordance with an embodiment of the invention; and

FIG. 8 depicts an exemplary discharging sequence for a string of safecells in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

Described herein are various embodiments of systems and methods foroptimally charging and/or discharging a subset of cells in a battery.Note that, as used herein, the term “cell” generally refers to a singleelectrochemical storage device, while the term “battery” generallyrefers to an array of two or more such cells. The term “safe cell”generally refers to a single cell and its associated circuitry, inaccordance with embodiments of the invention, for monitoring and/orcontrolling the cell. A “string of safe cells” (or, simply, a “string”or a “string of cells”) generally refers to a plurality of safe cellsconnected in series.

FIG. 1 is an illustrative embodiment of a safe cell 100 that includes apositive terminal 102 and a negative terminal 104. A cell 106 isconnected to a bypass switch 108 that selects between a first mode ofoperation, in which the cell 106 is connected in series between thepositive 102 and negative 104 terminals, and a second mode of operation,in which a bypass route 110 connects the positive terminal 102 directlyto the negative terminal 104. The cell 106 may be charged or dischargedin the first mode, while the cell 106 is disconnected and idle in thesecond mode. The bypass switch 108 may be implemented as any type ofswitch known in the art, such as a solid-state transistor-based switchor a relay.

The safe cell 100 may further include a monitoring device 112 thatmeasures the voltage across the cell 106. If the measured voltagedeviates from an acceptable range of values (e.g., lower than 2.4 voltsor greater than 4.1 volts for a lithium-polymer (“LiP”) cell), themonitoring device 112 may send a signal 114 to the switch 108 to bypassthe cell 106. In one embodiment, the monitoring device 112 is a voltagecomparator. An external connection 116 may receive the output of themonitoring device 112 and/or provide a control signal for the switch108.

FIG. 2 depicts an exemplary implementation 200 of the safe cell 100illustrated in FIG. 1. As illustrated in FIG. 2, a cell 202 is connectedin series with field-effect transistor (“FET”) cut-off switches 204 thatallow the cell 202 to be connected in series with positive 206 andnegative 208 terminals. A set of bypass FET switches 210 allow for adirect connection 212 between the positive 206 and negative 208terminals that does not include the cell 202. The cut-off and bypass FETswitches 204, 210 may be any type of semiconductor-based or mechanicalswitch. Two or more FET switches 204, 210 may be connected in series ineach path, as shown in FIG. 2, to improve reliability of the switches,or in parallel in each path to provide redundancy. In one embodiment,only one FET switch is used in the bypass connection 212 and only oneFET switch is used in series with the cell 202. The FET switches 204,210 may feature a very low series resistance at nominal current levels,but may be designed to limit current levels through the cell 202 atlevels much greater than nominal (e.g., five to ten times greater thannominal). This current limiting may protect the cell 202 against, forexample, an accidental short across the terminals 206, 208.

A voltage sensor 214 monitors the voltage across the cell 202 andoutputs a signal based on the monitored voltage to a comparator 216 thatcompares the monitored voltage to a reference value or values. A celllogic unit 218 observes the output of the comparator 216 and, if thevoltage of the cell 202 falls outside a safe range, as determined by thereference value(s), opens the series FET switches 204 and closes thebypass FET switches 210, thereby protecting the cell 202 against afurther deviation from the safe range.

A processor 220 may control the safe cell 200 and one or more additionalsafe cells 200 a-b, as explained further below. The processor 220 mayreceive the output of the voltage sensor 214. In one embodiment, theoutput of the voltage sensor 214 is converted to a digital signal by ananalog-to-digital converter (“ADC”) 222 before being passed to theprocessor 220. As described further below, the processor 220 may providea control signal 224 to the cell logic unit 218 that may instruct thecell logic unit 218 to open or close any of the FET switches 204, 210.

The processor 220 and the cell logic unit 218 may each be any digital,analog, or mixed-signal processing engine, including but not limited tomicroprocessors, microcontrollers, FPGAs, ASICs, and/or firmware. Someor all of the functionality of the processor 220 and/or the cell logicunit 218 may be implemented in software. The processor 220 and/or thecell logic unit 218 may be redundant or fault-tolerant units, so that ahardware failure therein does not compromise the operation and safety ofthe safe cell 200.

FIG. 3 is an illustration of a string 300 of safe cells 302-312 thatincludes cells 302 a-312 a and switches 302 b-312 b in accordance withembodiments of the invention. A subset of the cells 302-312 may bechosen (e.g., electrically coupled together) at any one time to providea desired output voltage across the output terminals 314, 316. Forexample, each cell 302-312 may be a two-volt cell, and the string 300may be required by an application to output six volts across itsterminals 314, 316. The string 300 may thus be configured to selectthree of the six cells 302-312, because three two-volt cells, combinedin series, provide six volts. In the embodiment depicted in FIG. 3,three cells 306 a, 308 a. 312 a (i.e., a non-contiguous group of cells)are connected in series with the terminals 314, 316. The selected cells306 a. 308 a, 312 a are non-contiguous at least because a cell 310 a islocated electrically within the selected cells 306 a, 308 a, 312 a butis not selected for inclusion therein. Note that the non-contiguousselected cells 306 a, 308 a. 312 a include two adjacent cells 306 a, 308a. Three other cells 302 a, 304 a, 310 a are bypassed using bypassswitches 302 b, 304 b, 31.0 b, respectively. As explained further below,the series-connected cells 306 a, 308 a, 312 a may be chosen duringbattery charging because they have the lowest charge of the six cells302 a-312 b. Alternatively, during battery discharging, theseries-connected cells 306 a, 308 a, 312 a may be chosen because theyhave the highest charge of the six cells 302 a-312 a.

In general, the string 300 includes at least one more cell than aminimum number of cells required to create a desired output voltage.Including additional cells in the string 300 may improve the lifetime,fault-tolerance, and total available charge of the string 300.High-power batteries, for example, may include hundreds of cells ofwhich only half are selected at any one time. The current invention,however, is not limited to batteries having any particular number ofcells. The cells 302-312 may be non-rechargeable or “primary” cells orrechargeable or “secondary” cells. Secondary cells may be charged and/ordischarged in accordance with embodiments of the current invention;primary cells may be discharged in accordance with embodiments of thecurrent invention.

In one embodiment, a failing cell 302-312 in the string of cells 300 ispermanently disabled (by, e.g., putting the cell in permanent bypassmode). Because non-contiguous cells may be combined to form a string ofcells (of any size) around the failing cell, a single failing cell doesnot affect the functionality of the string 300. As more cells fail,however, the string of cells 300 may provide less total capacity due tothe missing cells. The energy capacity of the string of cells may thusdegrade gracefully, allowing continued use while providing warning to anapplication using the string of cells that a complete failure of thestring of cells may be drawing near.

FIG. 4 is a flowchart 400 of an exemplary method for charging a batteryof cells. In brief overview, voltages of each of a plurality of cells inthe battery are determined (Step 402) and a subset of cells is selectedbased at least in part on the determined voltage (Step 404). The subsetof cells is then used to create a non-contiguous series connection ofcells (Step 406). The series connection of cells is then charged (Step408).

Referring to the first Step 402 in greater detail, a voltage V_(cell) ofeach cell in the battery may be determined using, for example, thevoltage sensor 214 described above with reference to FIG. 2. The cellvoltage V_(cell) may be measured when the cell is switched into thestring 300 or “on” (V_(cell) _(—) _(on)) and when the cell is switchedout of the string 300 and idle (V_(cell) _(—) _(off)). In oneembodiment, the cell is allowed to settle after switching for several(e.g., 5-20) seconds in its current state before a measurement is taken,thereby ensuring that the electrochemical properties of the cell havestabilized and improving the quality of the measurement. The currentrunning in the string of cells 300, I_(string), is also measured. Theimpedance of a given cell, Z_(cell), may then be computed according tothe equation:

$\begin{matrix}{Z_{cell} = \frac{V_{cell\_ on} - V_{cell\_ off}}{I_{string}}} & (1)\end{matrix}$

V_(cell) _(—) _(on) and V_(cell) _(—) _(off) may be re-measured eachtime each time a cell is connected or disconnected from the string 300,and Z_(cell) may be re-computed accordingly.

Once Z_(cell) has been computed, a current state of a cell may bepredicted using Z_(cell), even if the cell has been charged ordischarged since the last measurement of V_(cell). More specifically,the resting voltage of a connected cell, V_(rest), may be estimated bycomputing:

V _(rest) =V _(cell) _(—) _(on)−(Z _(cell) ×I _(string))  (2)

where V_(cell) _(—) _(on) in equation (2) is the measured voltage of thecell while the cell is switched into the string 300. In other words,because the cell's impedance Z_(cell) is known, the cell's restingvoltage V_(rest) may be estimated even when current is running throughthe cell. If the cell is disconnected, V_(rest) is simply the measuredcell voltage V_(cell) _(—) _(off). The resting voltage V_(rest) isequivalent to the Thevenin voltage of the cell.

Similarly, the operating voltage of a disconnected cell, V_(operating),may be estimated by computing:

V _(opening) =V _(cell) _(—) _(off)+(Z _(cell) ×I _(string))  (3)

where V_(cell) _(—) _(off) in equation (3) is the measured voltage ofthe cell when the cell is switched out of the string 300. If the cell isconnected, V_(operating) is simply the measured cell voltage V_(cell)_(—) _(on).

Once the cell voltages have been measured or estimated, a subset ofcells is selected based at least in part on the cell voltages (Step404). In one embodiment, one or more cells having the lowest measured orcomputed resting voltage V_(rest) is selected. In other embodiments, oneor more cells having the highest computed impedance Z_(cell) or thelowest state of charge is selected. In general, any method fordetermining the cell(s) in greatest need of charging may be used toselect the subset of cells. For example, referring to the string 300 ofcells 302-312 described above with reference to FIG. 3, the cells 306 a,308 a, 310 a may be determined to be the cells with the lowest voltagesV_(rest) and/or greatest impedances Z_(cell).

Optionally, after a cell is identified, that cell may be individuallycharged. In a typical charging system, however, the available chargingvoltage is greater than a maximum safe voltage for an individual cell.For example, a six-member string of two-volt cells may be used toprovide a six-volt power supply by selecting three of the six cells.Typically, a single two-volt cell may not be safely charged with asix-volt supply, because doing so may harm the cell. In addition,individually charging each cell in the string may take an unacceptablylarge amount of time and would be very inefficient.

Accordingly, once the subset of cells is identified in Step 404, aseries connection of cells is created in Step 406. The series connectionof cells may include any cells that require charging. Referring again toFIG. 3, for example, cells 306 a and 312 a are chosen to create a seriesconnection of cells 306 a, 308 a, 312 a. The switches 306 b. 308 b, 312b corresponding to the chosen cells 306 a, 308 a, 312 a are activated(by, for example, the processor 220) to connect those cell in serieswith the terminals 314, 316. The switches 302 b, 304 b, 310 bcorresponding to the non-chosen cells 302 a, 304 a, 310 a are activatedto bypass those cells. The created series connection of cells 306 a, 308a, 312 a is non-contiguous because it “skips” a cell 310 a within thebattery of cells.

In one embodiment, each of the cells is ranked according to a chargingcriteria (e.g., V_(rest), Z_(cell), and/or state of charge), and thecells are added to the series connection of cells based on their rank.For example, a desired battery terminal voltage V_(set) may be chosen(or may be specified by an application for use with the battery), andthe cells having the lowest resting voltages V_(rest) are added to theseries connection of cells until the sum of their operating voltagesV_(operating) is approximately equal to V_(set) (e.g., within apercentage equal to the inverse of the number of cells—within 10% ofV_(set) for a 10-cell battery or within 1% of V_(set) for a 100-cellbattery). As an example, cells may be safely charged by a range ofdifferent voltages, and V_(set) may lie within that range.

In step 408, the series connection of cells is charged. The voltagesacross the cells may be monitored during charging, and the cells in theseries connection may be removed from or added to the series connectionas necessary. For example, referring again to FIG. 3, a switch 302 b maybe toggled to include a corresponding cell 302 a in the seriesconnection of cells, and another switch 312 b may be toggled to removeits corresponding cell 312 a from the series connection of cells. In oneembodiment, the cell voltages V_(cell) _(—) _(on), V_(cell) _(—) _(off)are re-measured (i.e., the method returns to Step 402) after an intervalof time (e.g., 1 second to two minutes) has passed. The interval of timemay be determined by the total estimated charge time of the battery(e.g., between approximately 0.1 and 5% of the total charge time). Anapplication may require more precise charge equalization across thecells and thus choose a shorter interval of time between measurements(incurring an associated penalty in increased energy usage permeasurement). Alternatively, the application may reduce its poweroverhead by choosing a greater interval of time between measurements (atthe cost of possibly less-precise charge equalization across the cells).The interval of time may be derived from a cell's charging rate and adesired equalization goal (e.g., less than 1% difference in a state ofcharge in a cell in the series connection of cells and another cell inthe battery). Note that mismatched cells may contain a differentabsolute amount of charge but may be matched to other cells in thestring of cells based on a percentage of charge, wherein the percentageis equal to charge in the cell divided by the capacity of the cell. Inone embodiment, the combination and number of cells in the seriesconnection of cells is varied automatically at these intervals, ifneeded. In another embodiment, the combination and number of cells inthe series connection of cells is varied automatically to compensate fordifferences in a provided charging voltage. For example, if a chargingvoltage falls, the number of cells in the series connection of cells maybe reduced from twelve to ten.

Once a cell reaches its maximum charge V_(max), it is removed from theseries connection of cells that is being charged. Each cell may betested for reaching its maximum charge V_(max) at each time interval. Inone embodiment, the cells are tested for reaching their maximum voltageV_(max) asynchronously by, for example, local cell logic 218. As morecells reach their maximum voltage V_(max), the number of cells in theseries connection of cells may decrease. Because the lowest-chargedcells are continually identified and charged, all cells will be close toor at full charge when too few non-fully-charged cells are available toreach V_(set).

FIG. 7 illustrates an exemplary process 700 for charging cells in abattery in accordance with embodiments of the invention. Referring alsoto FIG. 3, in a first step 702, a battery of cells 302-312 is presentedfor charging; for this example, the operating voltage V_(operating) ofeach cell 302-312 is assumed to be 2.0 V and the charging voltageV_(set) is assumed to be 6.0 V. A cell 306 is determined to have thelowest resting voltage (V_(rest)=1.3 V). Additional cells 308, 312having the next-lowest resting voltages (V_(nest)=1.5 V, 1.6 V) areselected so that the sum of the operating voltages V_(operating) of theseries connection of cells 306, 308, 312 equals the charging voltage(V_(set)=6.0 V). The cells 306, 308, 312 are then charged and, after adelay, the voltages of the cells are determined again in a second step704. At this step, the voltage of the least-charged cell 306 hasincreased to 1.8 V, and the voltages of the other cells in the seriesconnection of cells has increased to a maximum voltage V_(max) of 2.0 V.Accordingly, a new series connection of cells is created to include newcells 304, 306, 310, with cell 310 now having the lowest resting voltage(V_(rest)=1.7 V). The new string of cells 304, 306, 310 is then charged,and, in a third step 706, it is determined that all cells 302-312 havereached their maximum voltage (V_(max)=2.0 V). Charging then ceases.

FIG. 5 is a flowchart 500 of an exemplary method for discharging abattery of cells. The method of discharging uses many of the same steps,measurements, and calculations described above with reference to FIG. 4.In a first step 502, individual cell voltages V_(cell) are determined,as is the resting voltage V_(rest) and operating voltage V_(operating)of each cell. In a second step 504, a subset of cells is selected basedat least in part on the determined voltages. For example, cells havingthe highest resting voltages V_(rest) (or lowest impedance Z_(cell) orhighest state of charge) are selected, and, in a third step 506, thesubset of cells is added to a series connection of cells until the sumof the operating voltages V_(operating) of the selected cells isapproximately equal to a desired battery output voltage V_(set) (e.g.,within a percentage equal to the inverse of the number of cells—within10% of V_(set) for a 10-cell battery or within 1% of V_(set) for a100-cell battery). In a fourth step 508, the cells in the seriesconnection of cells are discharged.

After an interval of time, the cell voltages V_(cell) _(—) _(on),V_(cell) _(—) _(off) are re-measured (i.e., the method returns to step502) and new cells, having higher resting voltages V_(rest) (or lowerimpedances Z_(cell) or highest state of charge), may be substituted oradded to the series connection of cells. A cell is removed from arotation of candidate cells when its voltage falls below a minimumvoltage V_(min). The battery is fully discharged when all cells havereached their minimum voltages V_(min) and/or when there is no longer acombination of available cells that can reach V_(set), wherein availablecells have a V_(rest) above V_(min). In one embodiment, differentcombinations or numbers of cells may be selected for the seriesconnection of cells to compensate for different loading requirementsplaced thereon.

As an example, referring to FIG. 8 and again to FIG. 3, cells 306, 308,312 are chosen in a first step 802 for discharging after havingdetermined that they have the highest resting voltages (V_(rest)=2.1 V,2.0 V, 2.0 V) and after having determined that the combination of thosecells satisfies a desired battery output voltage V_(set) of, forexample, 6.0 V (assuming that each cell 302-312 has an operating voltageV_(operating) of 2.0 volts). The cells chosen in the series connectionof cells 306, 308, 312 each drop 1.0 V in V_(rest) in a second step 804,at which point new cells 302, 304, 310 are chosen. These cells then drop1.0 V in a third step 806. If V_(min) is, for example, 0.5 V, the cells302-312 may be discharged still further in a similar manner. In oneembodiment, as the cells 302-312 discharge, more cells are added in theseries string.

Additional features may be used with any of the embodiments describedabove. For example, FIG. 6 illustrates a battery string architecture 600having voltage regulation. For rough voltage regulation, a processor 602can switch cells in a string 604 in or out of operation. For finevoltage regulation, the processor 602 may employ pulse-width modulationby opening and closing a string-disconnect switch 606. For a greatervoltage, the processor 602 may increase the amount of time per cyclethat the string-disconnect switch 606 is closed and, for a lesservoltage, decrease the amount of time per cycle. An inductor 612 may beadded to the string to improve the quality of the pulse-width-modulatedsignal. The processor 602 may sense the string current with a currentsensor 608 and the string voltage with a voltage sensor 610. Inaddition, the string 604 may include a fuse 620 to protect against anover-current condition.

One or more of the cells in a string of cells (for example, cells302-312 in string 300) may be mismatched in charge capacity whencompared to cells in the rest of the string. The mismatch may occur dueto manufacturing defects/variations or changes in a cell during itslifetime of use. The mismatch may also be deliberately introduced intothe string if, for example, cells from different manufacturers, ofdifferent capacities, or of different ages are combined. In oneembodiment, time-cycling a subset of cells in the string of cells (i.e.,periodically selecting different subsets of cells based at least in parton a determined voltage of each cell) accounts for any such mismatches.For example, a cell having less capacity may be selected for charging ordischarging for less total time (during the total charging ordischarging cycle) than a cell having more capacity. In this fashion,each cell in a string may be charged to its individual V_(max) (so thatthe voltage of every cell equals its V_(max) at the end of a chargingcycle) or discharged to its individual V_(min) (so that the voltage ofevery cell equals its V_(min) at the end of a discharging cycle). Duringcharging or discharging, the state of charge of each cell in the stringof cells may deviate across the cells by less than 1%; this cellequalization may be achieved by periodically selecting cells that havethe greatest deviation (e.g., having the highest or lowest V_(rest)) andcharging or discharging those cells.

When charging a string of cells, rough voltage regulation may first beused to select a number of cells that together provide an operatingvoltage V_(operating) as close as possible to a charging voltageV_(set). Then, fine voltage regulation may be used to remove anyremaining differences between V_(operating) and V_(set). For example,V_(set) may be 10.0 V and the closest possible V_(operating) may be 8.0V. In one embodiment, pulse-width modulation (i.e., fine voltageregulation) is used to modulate the output of an entire string of cellsto bring the 10.0 V charging voltage V_(set) down to a voltagecompatible with V_(operating) (e.g., 8.0 V). In another embodiment,pulse-width modulation is used to modulate the output of a subset ofcells in a string or even a single cell in the string. In thisembodiment, pulse-width modulation of a subset of cells in a string mayprovide more precise control of the string's output voltage. Roughand/or fine voltage regulation may be similarly used to adjust a stringvoltage to an exact value of a desired load voltage when discharging astring of cells.

In addition, embodiments of the current invention may be used to matchcharger and load requirements in certain applications. For example,solar voltaic arrays generally require a specific load voltage tomaximize their provided energy. Conventionally, this matching is doneusing a power-point tracker (“PPT”) that requires a DC-DC converter.Using the methods and systems described above, a string of smart cellsmay instead be configured to present a nearly ideal charging voltage tothe solar voltaic array.

As another example, a motor controller is most efficient when its supplyvoltage is slightly higher than the motor's back-electromagnetic force(“EMF”). In variable-speed drives, the supply voltage is typically setfor the motor's highest speed. At lower speeds, however, the controllergenerally reduces the voltage using pulse-width modulation. As analternative, a battery of cells in accordance with the current inventionmay be employed and its output voltage set to the optimal point for eachspeed, thereby offering a considerable motor controller efficiencyimprovement by eliminating the need for pulse-width modulation.

In another embodiment, again referring to FIG. 6, the stringarchitecture 600 may employ multiple strings 604, 614 connected to acommon power bus 616. High-power applications may include 10, 100, orany other number of multiple strings 604, 614. The multiple strings 604,614 may provide redundancy or back-up if one or more strings fail. Inone embodiment, multiple strings of lower-current cells are used tocreate a battery of cells having a large total output current.

Each string 604, 614 may be assigned a separate periodic time to performcell reordering during a charge or discharge cycle in order to reducetransients or glitches on the power bus 616 due to the switching of anumber of strings 604, 614. For example, a first string 604 may switchin or out any additional cells at a first time t=1, and a second string614 may switch in or out additional cells at a second time t=2. Eachstring 604, 614 may be disconnected from the power bus 616, using thestring switch 606, for a brief amount of time (e.g., a few milliseconds)until the string's switching has settled. This disconnecting may furtherreduce stress on the switch elements and reduce current disruptions to aload connected to the bus 616.

The terminal voltages of the strings 604, 614 may not exactly match,which may result in a new string having a voltage different from thevoltage on the power bus 616 causing a transient voltage to appear onthe bus 616 when the new string is connected. These transient effectsmay be detected and reduced or eliminated. Realizing the connectedstring's voltages may change due to charge or the load's currents, oneembodiment of a transient reduction method includes waiting for thecharging or discharging voltage of the string to match a connectedstring 604, 614 to match the voltage of a disconnected string 604, 614.Once the voltage of the strings match, the strings are togetherconnected to the bus 616. In another embodiment, the voltage of a string604, 614 that deviates from a common voltage is modulated (by, forexample, pulse-width modulation, as described above), so that it appearsequal to the common voltage.

The embodiments described herein may be used in a number ofapplications. For example, electrically powered vehicles such assubmarines (manned and unmanned), air vehicles (manned and unmanned),automobiles, and buses may employ the embodiments described herein.Homes and/or power grids that use systems of batteries for chargestorage may also employ the embodiments described herein. Furtherexemplary applications include peak-power-point tracking systems inarrays of solar cells. Essentially, the embodiments described herein maybe used in any application that employs one or more rechargeablebatteries.

Certain embodiments of the present invention were described above. Itis, however, expressly noted that the present invention is not limitedto those embodiments. Rather, the intention is that additions andmodifications to what was expressly described herein are also includedwithin the scope of the invention. Moreover, it is to be understood thatthe features of the various embodiments described herein were notmutually exclusive and can exist in various combinations andpermutations, even if such combinations or permutations were not madeexpress herein, without departing from the spirit and scope of theinvention. In fact, variations, modifications, and other implementationsof what was described herein will occur to those of ordinary skill inthe art without departing from the spirit and the scope of theinvention. As such, the invention is not to be defined only by thepreceding illustrative description.

What is claimed is:
 1. A method for charging a battery of cells,comprising: determining a voltage of each of a plurality of cells in thebattery; selecting a subset of the plurality of cells based at least inpart on the determined voltages; creating a non-contiguous seriesconnection of cells that includes the subset of the plurality of cells,the series connection of cells excluding at least one cell that requirescharging; and charging the series connection of cells with a chargingvoltage.
 2. The method of claim 1, wherein selecting the subset of theplurality of cells comprises determining at least one of a restingvoltage, impedance, or state of charge of each of the plurality ofcells.
 3. The method of claim 1, wherein the series connection of cellscomprises at least two adjacent cells.
 4. The method of claim 1, whereinan operating voltage of the series connection of cells is approximatelyequal to the charging voltage.
 5. The method of claim 1, furthercomprising modulating the charging voltage with a switching circuit. 6.The method of claim 1, further comprising re-determining, after a delay,the voltage of each of the plurality of cells.
 7. The method of claim 6,further comprising selecting, after the delay, a new subset of theplurality of cells.
 8. The method of claim 1, further comprisingidentifying a faulty cell in the battery and bypassing the faulty cell.9. The method of claim 1, further comprising monitoring, duringcharging, the voltage of each cell in the series connection of cells.10. The method of claim 1, wherein cells in the series connection ofcells are mismatched and a parameter of each cell deviates, duringcharging, by less than 1% across the cells.
 11. The method of claim 10,wherein the parameter is at least one of resting cell voltage, cellimpedance, and percentage of charge. 12-22. (canceled)
 23. A system formanaging charge on cells in a battery, the system comprising: avoltage-sensing circuit for sensing a voltage across each of a pluralityof cells; a processor for i) selecting a subset of the plurality ofcells based at least in part on the sensed voltage of each of theplurality of cells, and ii) causing the subset of the plurality of cellsto be electrically coupled in a non-contiguous series connection ofcells, the series connection of cells excluding, in a charging mode, atleast one cell that requires charging; and an output port across theseries connection of cells.
 24. The system of claim 23, whereinselecting the subset of the plurality of cells is further based on aresting voltage, impedance, or state of charge of each of the pluralityof cells.
 25. The system of claim 23, wherein the voltage-sensingcircuit comprises a voltage sensor local to each cell.
 26. The system ofclaim 23, wherein each of the plurality of cells further comprisescircuitry for disabling the cell if the sensed voltage deviates from athreshold.
 27. The system of claim 23, wherein each of the plurality ofcells further comprises a bypass switch.
 28. The system of claim 23,wherein the processor further comprises circuitry for selecting, after adelay, a new subset of the plurality of cells.
 29. The system of claim23, further comprising a pulse-width modulator for adjusting a voltageacross the output port.
 30. The system of claim 23, wherein the outputport is applied to a voltage source for charging the series connectionof cells.
 31. The system of claim 23, wherein the output port is appliedto a load for discharging the series connection of cells.
 32. A systemfor managing charge on cells in a battery, the system comprising: aplurality of cells, each cell being in parallel with a bypass switch andin series with a cutoff switch; a voltage-sensing circuit for sensing avoltage across each of the plurality of cells; and a processor for i)selecting a subset of the plurality of cells based at least in part onthe sensed voltages, and ii) configuring, using the bypass and cutoffswitches, a non-contiguous series connection of cells comprising thesubset of the plurality of cells, the series connection of cellsexcluding, in a charging mode, at least one cell that requires charging.